1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an alignment layer for initial alignment of liquid crystals in a liquid crystal display device.
2. Discussion of the Related Art
Among ultra-thin flat panel display devices with a display screen having a thickness of several □, liquid crystal display devices have been widely applied to notebook computers, monitors, space shuttles, aircrafts, and so fourth due to their advantages including low operating voltage, low power consumption, portability, and the like.
Generally, a liquid crystal display device comprises a color filter substrate having color filter layers formed thereon, a thin film transistor substrate facing the color filter substrate and having thin film transistors formed thereon, and a liquid crystal layer formed between these substrates.
In such a liquid crystal display device, alignment of the liquid crystal layer is varied by application of voltage to control transmittance of light, thereby allowing an image to be reproduced. Thus, electrodes are formed on the thin film transistor substrate and/or the color filter substrate for application of the voltage such that a pixel electrode is located on the thin film transistor substrate and a common electrode is located on the color filter substrate so as to create a vertical electric field between the two substrates, or such that the pixel electrode and the common electrode are located parallel to each other on the thin film transistor substrate so as to create a horizontal electric field.
FIG. 1A is an exploded perspective view illustrating a liquid crystal display device of the former case, and FIG. 1B is an exploded perspective view illustrating a liquid crystal display device of the latter case, both according to a related art.
In the liquid crystal display device shown in FIG. 1A, a thin film transistor substrate 10 has a gate line 12, a data line 14 crossing the gate line 12, a thin film transistor T formed on a crossing region of the gate line 12 and the data line 14, and a pixel electrode 16 connected to the thin film transistor T. A color filter substrate 20 has a light shielding layer (or black matrix) 22 formed thereon in order to prevent leakage of light, color filter layers 24 for red, green and blue light, that is, R, G and B, formed in the light shielding layer 22, and a common electrode 25 formed under the color filter substrate 20. In this manner, a vertical electric field is created between the pixel electrode 16 on the thin film transistor substrate 10 and the common electrode 25 on the color filter substrate 20, thereby allowing alignment of liquid crystals to be controlled.
In the liquid crystal display device shown in FIG. 1B, a thin film transistor substrate 10 has a gate line 12, a data line 14 crossing the gate line 12, a thin film transistor T formed on a crossing region of the gate line 12 and the data line 14, and a pixel electrode 16 connected to the thin film transistor T. Additionally, a common electrode 25 is formed parallel to the pixel electrode 16 on the thin film transistor substrate 10. A color filter substrate 20 has a light shielding layer 22 formed thereon for preventing leakage of light, color filter layers 24 for red, green, and blue light, that is, R, G, and B, formed in the light shielding layer 22, and an overcoat layer 27 formed under the color filter substrate 20 for flattening the substrate. In this manner, a horizontal electric field is created between the pixel electrode 16 and the common electrode 25 on the thin film transistor substrate 10, thereby enabling alignment of liquid crystals to be controlled.
Both substrates 10 and 20 constructed as described above are combined to form a single liquid crystal display panel, and have a liquid crystal layer formed therebetween. At this time, if the liquid crystal layer is randomly aligned between the substrates 10 and 20, it is difficult to achieve a consistent arrangement of molecules in the liquid crystal layer. Thus, although not shown in the drawings, an alignment layer for the initial alignment of liquid crystals is formed in the thin film transistor substrate 10 and/or the color filter substrate 20.
The alignment layer is generally formed by a rubbing alignment method as known.
The rubbing alignment method is a method in which, after an organic polymer such as polyimide is coated in the form of a thin film on a substrate, the organic polymer is rubbed by rotating a rubbing roll having rubbing fabrics wound thereon, thereby arranging side chains of the organic polymer in a constant direction. The liquid crystals are aligned in a direction in which the side chains of the organic polymer are arranged, and a moving direction of the rubbing roll becomes the alignment direction of the liquid crystals.
However, the rubbing alignment method has several drawbacks as described below.
First, when the arrangement of the rubbing fabrics becomes disordered, a problem of light leakage can occur. FIG. 2 is a schematic perspective view illustrating a disordered arrangement of the rubbing fabrics.
As described above, since the components such as the thin film transistor, the color filter layer and the electrode layers are formed on the substrate (see FIGS. 1A and 1B), some portion 32a of the rubbing fabrics 32 wound around the rubbing roll 30 can become disordered when the rubbing roll 30 rotates on the components formed on the substrate 10 or 20 as shown in FIG. 2. As such, when the arrangement of the rubbing fabrics becomes disordered, the side chains of the organic polymer in a region rubbed by the disordered rubbing fabrics cannot be aligned, resulting in light leakage in that region due to non-uniform alignment of the liquid crystals.
Second, when the rubbing fabrics do not contact the substrate, the problem of light leakage can occur. FIG. 3 is a schematic perspective view illustrating an alignment state of the liquid crystals when the rubbing fabrics do not contact the substrate.
As described above, the electrode layers such as the pixel electrode and common electrode are formed on the substrate (see FIGS. 1A and 1B). Thus, as shown in FIG. 3, a region A where the rubbing fabrics 32 do not contact the substrate due to a step on the substrate 10 is formed. In this case, the alignment of the liquid crystals is not uniform in the region A, thereby creating the problem of light leakage.
In particular, in the liquid crystal display device shown in FIG. 1A, since the pixel electrode and the common electrode are formed in pixel regions on different substrates, respectively, there may not be so many regions having the steps formed thereon. However, in the liquid crystal display device shown in FIG. 1B, since the pixel electrode and the common electrode are repetitiously formed in parallel to each other in pixel regions on the substrate, there are many regions having the step formed thereon, whereby the problem of light leakage becomes serious.
As such, the problems in the rubbing alignment method according to the related art are caused by the mechanism for providing physical contact between the rubbing roll and the substrate.
Recently, in order to solve these problems of the rubbing alignment method, various studies have been conducted for providing a method for manufacturing an alignment layer which does not require physical contact. In particular, instead of using the rubbing alignment method, use of a photo-alignment method has been suggested, in which an alignment layer is produced by irradiating polarized ultraviolet rays onto a polymeric film. In order to align the liquid crystals, the alignment layer must have an anisotropic structure, which can be formed when the polymeric film is anisotropically reacted with the polarized UV ray.
However, although the photo-alignment method may address the above-described problems related to the rubbing alignment method described above, the photo-alignment method has a serious problem in that anchoring energy is low. More specifically, with the rubbing alignment method, since the side chains of the organic polymer are arranged in the constant direction as described above and grooves are uniformly formed over the surface of the substrate by rubbing, the alignment of the liquid crystals is controlled by mechanical interaction between the grooves and the liquid crystals as well as by chemical interaction between the side chains and the liquid crystals. On the contrary, with the photo-alignment method, the alignment of the liquid crystals is controlled by the chemical interaction between the side chains and the liquid crystals caused only by the photo reaction without forming the grooves on the surface of the substrate. Accordingly, in comparison to the rubbing alignment method, the photo-alignment method provides the lower anchoring energy and causes a problem of image sticking.
Since the problem of image sticking by the photo-alignment method is serious to such an extent that the method cannot be applied to large-scale production lines, the rubbing alignment method has been used for a large production line in spite of the problems with the light leakage.
As liquid crystal display devices of a higher quality have been increasingly required, there is a need for developing a method of aligning the liquid crystals, which can overcome or minimize the problems of the rubbing alignment method and the photo-alignment method according to the related art.